1. Field of the Invention
The invention relates to a heat-shielded vehicular exhaust muffler.
2. Description of the Related Art
The combustion that takes place in the engine of an automotive vehicle produces substantial amounts of heated noxious gas and significant amounts of noise. As a result, all automotive vehicles include an exhaust system that transports the exhaust gas from the engine to a location on the vehicle where the heat exhaust gas can be emitted safely. Additionally, the exhaust system includes components to convert certain of the noxious compounds in the exhaust gas into less noxious gases. Components of the exhaust system also function to attenuate the noise associated with the flowing and rapidly expanding gases produced by the combustion processes in the engine.
The typical exhaust system extends from the engine compartment near the front of the vehicle to a location at or near the rear of the vehicle where the exhaust gases may be emitted safely. The exhaust system includes a plurality of pipes, a catalytic converter and at least one muffler. These various components of the vehicle must compete for space on the underside of the vehicle with other necessary components of the vehicle. The muffler typically is the largest component of the exhaust system and hence is the most difficult to place on the vehicle. Stamp forming technology allows the designers of an exhaust system freedom to choose an appropriately configured muffler that can be nested into a space on the underside of the vehicle.
The entire exhaust system becomes very hot after a short period of operation due to the high temperatures generated during the combustion processes that produce the exhaust gas. The realities of designing an exhaust system to fit into the limited space on the underside of a vehicle typically positions certain components of the exhaust system close to passenger compartments, luggage compartments or other heat sensitive components or sections on the vehicle. As a result, most exhaust systems must include at least one heat shield, including a heat shield near the muffler.
The typical heat shield for a muffler is a thin sheet of metal that is stamped or otherwise formed to conform generally to the shape of the muffler. The heat shield may be formed with legs or other structures that provide small areas for attaching the heat shield to the muffler. However, a major portion of the typical heat shield is spaced from the outer shell of the muffler to provide an air gap that will insulate sensitive areas of the vehicle from the heated muffler. The heat shield typically is secured to the muffler by welding. However, other attachment means, such as straps, rivets or folded seams have been employed in the prior art.
Heat shields can be designed to perform their primary heat shielding function adequately. However, the metal of the heat shield adds to the cost and weight of the exhaust system. In this regard, automobile manufacturers exert substantial pressure on suppliers to reduce the size and weight of their products to enhance the fuel efficiency of the vehicle and to maximize space available for other components of the vehicle. Additionally, the automotive industry is extremely competitive and suppliers to the automotive industry are constantly looking for cost savings. Even small cost reductions can have a substantial commercial advantage.
The prior art heat shields also create the potential for maintenance problems. In particular, parts of the heat shield necessarily must be spaced from the muffler to perform the heat shielding function. As a result, the heat shield is substantially cooler than adjacent areas of the muffler. The temperature differential between the heat shield and the muffler leads to differential thermal expansion. Therefore, the weldments or other such attachments between the heat shield and the muffler are subject to substantial and repeated forces as the muffler goes through its heating and cooling cycles. Additionally, the entire exhaust system is subject to significant vibration during use. Consequently, the welded attachments between the heat shield and the muffler are subject to failure. A failed connection will cause the heat shield to vibrate against the exterior of the muffler and/or against other nearby parts of the vehicle. Such vibrations can create very objectionable noise. A folded connection between the heat shield and the muffler can be designed to accommodate some motion during differential thermal expansion without adversely affecting the long term connection between the muffler and the heat shield. However, folds or other such mechanical connections also are subject to vibration during use and hence can generate objectionable noise.
The muffler of an exhaust system includes an outer shell with at least one inlet that connects to an exhaust pipe and at least one outlet that connects to a tail pipe. The interior of the muffler includes an array of tubes and/or baffles that are designed to permit a controlled expansion of the exhaust gas in a manner that will attenuate the noise associated with the flowing exhaust gas. Some mufflers include conventional tubular pipes that are supported by transverse baffles in the muffler. The baffles define chambers within the muffler and the pipes are disposed to provide communication from one chamber to another. Other mufflers include stamp formed internal plates to define the exhaust gas channels and baffles within the muffler. Some chambers within some mufflers are filled with a loose array of fibers, such as fiberglass or E-glass. The array of fibers fill the chambers, but are sufficiently loosely arrayed to permit the exhaust gas to expand in the chamber and flow through the array of fibers. The array of fibers contributes to the noise attenuation function of the internal tubes and chambers of the muffler.
In view of the above, it is an object of the subject invention to provide a muffler to achieve effective heat shielding without the above-described problems associated with external mounted metallic heat shields.
It is another object of the subject invention to provide a heat shielded muffler without the cost, size and weight penalties associated with an externally disposed metal member.
An additional object of the subject invention is to provide a heat shielding arrangement for a muffler that is not likely to create vibration related noise.